1. Field of the Invention
The present invention relates to a method of forming the electrode of a semiconductor device, and more particularly to a method which can form a good electrode on a silicon wafer.
2. Description of the Prior Art
As is well known, the electrodes of various semiconductor devices are formed principally by employing an Al alloy as an electrode material or by producing metal silicide directly on a silicon substrate.
The method employing the Al alloy as the electrode material has the merit that the process is simple and that the manufacture is easy. Since, however, the Al alloy is readily alloyed with Si at a low temperature of about 500.degree. C., Al penetrates into the Si substrate on account of the solid-phase diffusing reaction between Si and Al. In case a diffusion layer in the Si substrate is thin, the Al having penetrated pierces the diffusion layer easily, resulting in the disadvantages that the junction shorting takes place and that the heat resistance is conspicuously low.
The method which forms the electrode by forming metal silicide directly on the Si substrate, is more excellent in the heat resistance than the method which employs the Al alloy.
In forming the electrode, however, the metal silicide is formed by the solid phase reaction between a metal and the Si substrate, so that when it is intended to form a thick metal silicide layer, the metal silicide in a lower part thereof enters the Si substrate into the state in which the interface between the metal silicide and the corresponding part of the Si substrate lies within the original Si substrate.
Within the Si substrate, regions opposite in the conductivity type to the Si substrate are usually formed. Therefore, when the interface is formed within the Si substrate, a junction is feared to be ruined by the metal silicide, so that the thickness of the metal silicide cannot be made very great.
In order to solve this problem, there has been proposed a method in which a polycrystalline Si film heavily doped with an impurity is formed on the Si substrate in advance, and a metal film is deposited thereon to react it with the polycrystalline Si film, thereby to form the metal silicide.
Owing to the presence of the polycrystalline Si film, this method can prevent the junction within the substrate from being ruined. Since, however, a polycrystalline Si film needs to be formed in parts to become contact holes by the use of the photoetching process, the method is unsuitable for the formation of a semiconductor device of high packaging density having a very fine structure.